The proposed H.264 video coding includes an I_PCM (intra-frame pulse code modulation) macroblock coding mode. While video compression techniques attempt to compress every macroblock of a video sequence, there is no guarantee that individual macroblocks will in fact be compressed. In practice, white noise will be expanded to some extent by compression techniques.
The I_PCM mode in the current H.264 specification provides a coding mode that guarantees a limit on the expansion of white noise during compression. The I_PCM mode provides a mechanism for an encoder to potentially more easily produce a bistream that guarantees a maximum number of bits per macroblock, thereby potentially enabling simpler decoding hardware that takes advantage of the guaranteed limitation.
The I_PCM mode generates the actual values of the pixels contained in a 16×16 macroblock, rather than attempting to compress the information. The I_PCM mode is a type of “fail safe” mode that bounds the size of a macroblock in the compressed video bitstream. While PCM coding has been implemented in the past, H.264 is the first instance where an I_PCM macroblock mode is incorporated into a video encoder/decoder (CODEC) that switches between PCM and non-PCM coding modes.
The current H.264 proposal does not provide a provision to bypass the context adaptive arithmetic entropy encoding (CABAC) stage for I_PCM mode encoded macroblocks. An idea behind the I_PCM mode is to avoid all compression. Therefore, it would be beneficial to bypass the entropy encoding stage for bits that belong to an I_PCM mode macroblock.
Solutions to the above problem existed in some early versions of the H.264 standard. The current H.264 solution is that when context-based adaptive binary arithmetic coding (CABAC) is terminated, the next bit to be decoded must be a single specific syntax element (i.e., RBSP_STOP_ONE_BIT). For example, if an offset value (i.e., CODIOFFSET) is larger than or equal to a range value (i.e., CODIRANGE), a value of 1 is assigned to a value (i.e., BINVAL), no renormalization is carried out and the CABAC encoding is terminated. In such a case, the last bit inserted in register RBSP_STOP_ONE_BIT is the offset value CODIOFFSET.
The disadvantage of such an approach is a lack of an ability to terminate the CABAC encoding prior to sending an I_PCM mode macroblock unless the slice being encoded is also terminated at the same time. For an I_PCM mode macroblock, the next bit decoded after terminating the CABAC encoding could be either a syntax element (i.e., PCM_ALIGNMENT_ZERO_BIT), or a first bit of the syntax element (i.e., PCM_BYTE).
The existing solutions disallow CABAC encoded slices that contain any I_PCM mode macroblocks. To use the I_PCM mode with the existing solutions, the current slice must first be terminated, and then a new slice begun with an I_PCM mode macroblock.
There are typically many bits of overhead associated with terminating an existing slice and beginning a new slice since a new slice header must be transmitted. For broadcast applications in which the overhead bits needed by many small slices are not needed for error resilience (i.e., with internet streaming applications) the existing approaches carry an undesirable penalty in overhead.
It would be desirable to implement a method and/or circuit that effectively bypasses the entropy encoding stage in an H.264 compliant CODEC for bits that belonged to an I_PCM mode macroblock.